This invention relates to markings for toric contact lenses. The markings are useful for identifying the offset between the cylindrical axis and the ballast axis.
Contact lenses having a toric optical zone (commonly referred to as xe2x80x9ctoric contact lensesxe2x80x9d) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens).
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. For example, a section of the lens periphery may be thicker (or thinner) than another section to provide the ballast. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast. Accordingly, toric contact lens prescriptions specify this offset, with toric lenses generally being offered in 5 or 10-degree increments ranging from 0xc2x0 to 180xc2x0.
U.S. Pat. No. 4,976,533 discloses various prior methods for marking a toric contact lens, the markings being arranged in a manner that the ballast axis is identifiable. As an example, FIG. 2 of U.S. Pat. No. 4,976,533 illustrates a prior art toric contact lens including three visible line segments in a peripheral section of the lens, a first line segment lying on the ballast axis and two line segments arranged symmetrically about the first. As another example, FIGS. 3 and 5 of U.S. Pat. No. 4,976,533 disclose one or two line segments lying perpendicular to the ballast axis. Such lenses are useful for measuring the rotation of the lens on the eye, i.e., a practitioner can evaluate the lens when placed on the eye, and use the markings to measure any deviation of the lens from its intended rotational position, and to evaluate whether rotation of the lens on the eye is effectively inhibited as intended.
The invention relates to markings for toric contact lenses that include a posterior surface and an anterior surface, one of the surfaces including a toric optical zone having a cylindrical axis, and the surfaces being shaped to form a ballast oriented about a ballast axis. The lens surfaces include two markings, a first identifiable marking that is aligned with the cylindrical axis, and a second identifiable marking that is aligned with the ballast axis. The two markings provide a simple and reliable means to identify the offset between the cylindrical axis of the toric optical zone and the ballast axis.
The invention also provides a method for marking a toric contact lens with identifiable markings, that comprises applying a first identifiable marking to a surface of the contact lens that is aligned with the cylindrical axis, and applying a second identifiable marking to a surface of the contact lens that is aligned with the ballast axis.